1. Field of the Invention
The present invention relates to signaling to set up, maintain and tear down circuits in circuit switched networks, such as the public telephone system, and virtual circuits in packet switched networks, such as for voice over the Internet Protocol (VoIP); and, in particular, to using Internet Protocol (IP) routers to replace signal transfer point (STP) routers used in legacy telephone systems or SIP based transfer functions within Call State Control functions (CSCF) in VoIP.
2. Description of the Related Art
Networks of communications devices and general-purpose computer systems connected by external communication links are well known and widely used in commerce. The networks often include one or more network devices that facilitate the passage of information between end stations, such as telephones and general purpose computing devices, that originate or receive the information. A network node is a network device or end station connected by the communication links. Information is exchanged between network nodes according to one or more of many well known, new or still developing protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other based on information sent over the communication links.
Legacy telephone systems utilize a network of switches connected by communication links including twisted pair copper wire and large capacity trunk lines. Various telephone devices are connected directly or indirectly to these switches. When a call is made, multiple switches in the network are configured to provide a complete circuit between two or more calling and called parties. Such a network is called a circuit-switched network. Signaling information, as used herein, is data that indicates to each switch what connections to make internally to establish, maintain or tear down a circuit between calling and called parties. In some systems using in-band signaling, the communication links used to connect called and calling parties are also used to communicate signaling information. As networks increased in size and a menu of telephone system functions increased (e.g., call forwarding, voice mail, toll free long distance, etc.), a separate network of signaling devices becomes effective. Thus in larger and more modern legacy telephone systems, out-of-band signaling is used, in which separate signaling devices communicate with each other over different communication links devoted to signaling. The out-of-band signaling uses one signaling device to control multiple switches, and not only diverts traffic from the main communication lines, but also sets up a typical circuit by sending less signaling information between signaling devices than would have been sent among the switches themselves.
Common Channel Signaling System 7 (SS7) is a set of standards that define the protocols and procedures for exchanging information between signaling devices in a signaling network. In effect, the SS7 network of signaling devices functions as a control center that governs all signaling network services and functions. Those functions have expanded over the years to include subscriber authentication, telephone number portability, mobile phone location, short message service (SMS), and other data services.
The signaling devices using SS7 are called herein, SS7 nodes. A current widely deployed commercial SS7 node is called a signal transfer point (STP). The STP is connected to multiple switches in the circuit-switched network. A signaling control point (SCP) is a node in the circuit-switched network that contains a service database and application software to provide one of the expanded services. An SCP is also connected to an STP. An STP receives a request from a switch for a connection and notifies the next switch in the circuit to be established for the call how to make its connections. The STP inserts any logic and checks any SCP and other system databases needed to determine or inform the next switch how to make the connections in consonance with the services offered by the call provider. The STPs communicate with switches, each other and with SCPs using signaling links that are set by the SS7 standard at 56 kilobits per second (Kb/s, where 1 Kb is on the order of 103 bits, actually 1024 bits, and a bit is a binary digit). This limit is easily exceeded in modern networks, so the standard was extended to include high speed links (HSL) that allow about fourteen (14) times this speed, and time division multiplexing (TDM) that allows signals to alternately use the same communication links.
A packet switched network (PSN) uses multi-purpose physical connections between adjacent nodes in a network to send limited-sized packets of data among connected nodes. At each intermediate node a decision is reached about which communication link to forward the packet received, if any. A long message comprises a series of packets that may be routed differently among nodes based on the available and less congested connections. A particular physical connection is used by packets from many different messages and is not reserved for a particular combination of communicating parties. The flexibility and robustness of PSNs has led to their wide adoption. In particular, the Internet Protocol (IP) has gained wide acceptance for communicating data, including voice and video data, between far flung end nodes on a PSN. PSN routers and switches are widely used in public and private networks, and can often be obtained and operated more cheaply than STP devices. IP communicates over Ethernet links that are capable of data rates from Megabits per second (Mb/s, 1 Mb is on the order of 106 bits) to ten Gigabit per second (Gb/s, 1 Gb is on the order of 109 bits).
A recent approach is to use lower cost, higher speed and more widely available IP devices, rather than STP devices, to handle some signaling traffic. Special protocols for sending signaling data packets over IP have been developed, including SCTP, and also including M2PA, M3UA and SUA for carrying various layers of the original SS7 protocol stack, MTP3, SCCP, and TCAP, respectively, inside SCTP payloads. These acronyms are defined in a later section, with reference to FIG. 2. With the shift in the industry direction away from TDM based signaling towards IP based signaling, some signaling routers such as upgraded STPs provide not only traditional MTP/SCCP layer routing but also routing at the IP layer (e.g. BGP, OSPF).
A relatively low cost IP gateway device translates between STP signals and IP data packets. For example, IP Transfer Point (ITP) switches, available from Cisco Systems of San Jose, Calif. serve as gateway routers between STP nodes and PSN nodes. More and more SCPs and switches are being deployed with IP compatible signaling links (e.g., Ethernet) to take advantage of an IP network between the switch, the SCP and the STP.
While suitable for many purposes and commercially deployed in many networks, there is a disadvantage in the approach of using an STP device with gateway devices to translate STP signals to IP data packets. The STP devices are expensive to maintain and upgrade. Much of the expense lies in the processing of the special purpose legacy network protocols that are redundant in function with the more widely used and lower cost to implement IP protocol. Furthermore, the STP routing protocol MTP3 has a limited set of options compared to IP routing. Legacy STP devices typically cannot route at the IP layer themselves.
IP routing technologies include many existing, and developing processes, including, among others, different routing protocols, including static routing, Border Gateway Protocol (BGP), Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), and Multiple Protocol Label Switching (MPLS). IP technologies also include different treatment of packets to be routed, including quality of service, tunneling using MPLS or a layer 2 protocol, virtual private networks, network address translation (NAT), packet encryption in IPsec, IP version 6 addresses, traffic filtering, access control lists, policy based routing, Hot Standby Routing Protocol (HSRP), Next Hop Resolution Protocol (NHRP), and any transport over MPLS (AToM). IP technologies continue to grow with time. Most of these options are not available in MTP of the original SS7 standard.
Upgrades for expanded services at the STPs are expensive because their costs are leveraged over a relatively small specialized community of STP users. As networks expand, ever more STPs are procured, and each purchase is typically matched with an IP gateway device. Yet the combination fails to use the full spectrum of IP routing options.
Based on the foregoing, there is a clear need for techniques that provide SS7 signaling for circuit switched networks that do not suffer one or more of the disadvantages of current systems using an STP paired with an IP gateway device.